Authorities in southern India put police on high alert Monday, fearing an outbreak of civil unrest after one of the country's most popular leaders Jayalalithaa Jayaram suffered a cardiac arrest.

Doctors treating the 68-year-old former film star, who enjoys an almost god-like status in the southern state of Tamil Nadu which she rules as chief minister, said she was in critical condition and had been admitted to intensive care.

Hundreds of supporters have kept a round-the-clock vigil outside the private Apollo hospital in the state capital Chennai since she was first admitted in September suffering from a fever.

On Monday television footage showed crowds praying for the chief minister, who is known simply as 'Amma', many of them in tears.

"Amma should survive. I won't mind if my life is taken away, but Amma should live," one supporter cried.

Jayalalithaa has not been seen in public since September, but the hospital had said her health was improving before she suffered a cardiac arrest on Sunday evening, sparking fresh concerns.

The Press Trust of India said police across the state had been put on high alert to maintain law and order, with 1,000 officers stationed at the hospital alone.

Media reports said some Chennai schools would remain closed, while the United States consulate in the city said it had suspended services and warned Americans to exercise caution.

There were no immediate reports of unrest, but several of her supporters resorted to self-harm when she was briefly jailed in 2014 on charges of corruption.

Jayalalithaa earned the loyalty of many voters in Tamil Nadu with a series of highly populist schemes including an 'Amma canteen' that provides lunch for just three Indian rupees.

In 2014 she was briefly forced to step down as chief minister after she was jailed on corruption charges.

Her conviction, which was later overturned on appeal, sparked mass protests and even some reported suicides.

Thousands of directors, actors and producers in the successful Tamil language film industry went on hunger strike to demand her release.